[clang] [flang] [clang][driver] When -fveclib=ArmPL flag is in use, always link against libamath (PR #116432)

[Tom Eccles via cfe-commits]

https://github.com/tblah approved this pull request.

This looks good to me but please don't merge without agreement on this approach from the other reviewers.

https://github.com/llvm/llvm-project/pull/116432